1. Field of the Invention
This invention relates to conductive, electrocatalytic coatings, such as electrocatalytic mixed oxide coatings and to stable, coating solutions for preparing mixed oxide coatings on metal substrates. The mixed oxide coatings on metal substrates or metal coated conductive substrates which are prepared utilizing the stable coating solutions of the invention are suitable as dimensionally stable anodes in electrolytic processes.
2. Description of Related Prior Art
The discovery of dimensionally stable anodes represents an important step in the progress of industrial electrolytic chemistry over the last thirty years. The advantages offered by dimensionally stable anodes have been exploited in various electrochemical processes including cathodic protection, electro-organic oxidations, and electrolysis of aqueous solutions. Because of the industrial importance of the electrolysis of aqueous solutions, the improvement disclosed herein relating to stable coating solutions useful in the preparation of dimensionally stable anodes will be described, particularly, with respect to the electrolysis of aqueous solutions, preferably, the electrolysis of alkali metal halides such as sodium chloride brine for the production of chlorine, caustic soda, and hydrogen.
The dimensionally stable anodes disclosed by Martinsons in U.S. Pat. No. 3,562,008 can comprise a valve metal base such as titanium having a coating of a thermally-decomposable titanium compound and a thermally-decomposable noble metal compound. The coating compounds are heated to decompose them to the oxides in order to prepare the mixed oxide coating on the valve metal base.
Beer in U.S. Pat. Nos. 3,711,385 and 3,632,498 discloses dimensionally stable anodes and liquid coating solutions for use in applying, respectively, soluble compounds of at least one platinum group metal or soluble metal compounds of at least one platinum group metal and a film-forming metal to a valve metal base in the preparation of an electrode for use in an electrolytic process. Beer et al. in U.S. Pat. No. 4,797,182 have sought to improve the lifetime of dimensionally stable electrodes having a film-forming metal base by the use of multiple, separate component layers of platinum metal and an oxide of iridium, rhodium, palladium, or ruthenium.
Bianchi et al. in U.S. Pat. No. 3,846,273 disclose doping a valve metal oxide base to provide electrodes having semi-conductive surfaces. These surfaces are produced on a valve metal base such as titanium or tantalum by applying a soluble mixture of metal compounds in several separate layers and heating the coating on the valve metal base between the application of each layer. Methods of producing the electrodes of '273 are disclosed in U.S. Pat. No. 4,070,504. Bianchi et al. in U.S. Pat. No. 4,395,436 disclose a process for preparing a dimensionally stable electrode by the application on a valve metal substrate of a metal compound capable of decomposing under heat. The coating is thereafter subjected to localized high intensity heat sufficient to decompose the compound while maintaining a portion of the substrate at a lower temperature.
The various coating compositions disclosed in the above prior art references, fail to address the problem of the long term stability of the coating solutions used to apply these coatings to a valve metal substrate. The stability of the coating solution for preparing the electrode is of less importance where the components of the coating solution are merely soluble ruthenium and titanium compounds. Where it is found necessary to use a soluble iridium compound in addition to ruthenium and titanium compounds, the substantially greater cost of the iridium compounds mandates that the coating solution have long term stability. It has been found to be desirable to have coatings of mixed oxides, for instance, iridium oxide in admixture with ruthenium oxide and titanium oxide in the catalytic coating in order to provide an anode having a longer lifetime than has been demonstrated for the prior art mixed oxide ruthenium oxide and titanium oxide catalytic coating on valve metals.
In general, an early failure of such electrodes is attributed to two major factors, namely, loss of the active coating by dissolution, and/or, passivation by the formation of a highly resistive TiO.sub.2 or Ta.sub.2 O.sub.5 layer between the substrate and the oxide coating. Sometimes these two factors occur simultaneously and the electrode at the end of its lifetime may show some active material left in the coating or may show a substantial coating amount remaining but passivation has occurred so as to require that the anode be operated at increased potential. In accordance with the teaching of U.S. Pat. No. 4,797,182, a common solution to the problem of loss of the active component in the coating and subsequent passivation of the substrate is the use of thicker coatings of the active component. For instance, 10 to 20 layers of the active coating provide an increased lifetime in electrodes utilizing the same coating composition. It is obvious that an increase in the coating thickness results in a significantly increased cost for the electrode.
Dimensionally stable anodes based upon a catalytic coating on a valve metal substrate have been used in diaphragm cells, mercury cells, and in membrane cells. A significant difference in anode coating life results from differences between the operation of these types of cells. For instance, a different current density characterizes each of these cells. Diaphragm cells are designed to operate at current densities of about 0.4-1.0 ASI while membrane cells typically operate at 2.0-3.0 ASI. Mercury cells typically operate at about 6.5 ASI. Other factors which influence anode coating life include the coating formulation and the operating parameters including brine purity, and exposure to alkaline operating conditions. In diaphragm cells, the diaphragm typically is in contact with the anode, and where the diaphragm swells excessively, alkaline exposure of the anode occurs. Still another factor affecting coating life is the production of oxygen under acidic conditions. Membrane and diaphragm cells always produce a small amount of oxygen at the anode on the order of 0.5-3.0 volume percent as a result of inefficiency reactions. In membrane cells which operate at a substantially higher current density than diaphragm cells, the amount of oxygen produced at the anode per unit time is significantly higher than for diaphragm cells. In addition, damage to the cell membrane often exposes the anode to a highly alkaline environment which causes rapid degradation of the anode.
In the dimensionally stable anodes for chlor-alkali electrolytic cells as disclosed in the Beer patents, referred to above, which contain a catalytic coating on a titanium base consisting of a mixture of ruthenium and titanium oxides, one cause of anode degradation is the formation of RuO.sub.4 during oxygen evolution at the anode. While oxygen evolution is only about 1-3 percent of the chlorine evolution at the anode during normal cell operation, the long term effect of the formation of RuO.sub.4 is significant. Accordingly, it would be desirable to develop an anode having a longer lifetime in which oxygen evolution at the anode is suppressed by modifying the chemical composition of the catalytic anode coating.